1. The Field of the Invention
The present invention relates to catalysts and methods for selective hydrogenation of nitro groups of halonitro aromatic compounds.
2. The Relevant Technology
Aromatic amines are important intermediates in the manufacture of dyes, drugs, herbicides and pesticides. It is known that halonitro aromatic compounds may be reduced to amino aromatic compounds in good yields in the presence of noble metal catalysts and hydrogen (i.e., in a hydrogenation process).
Substantial efforts have been exerted to find conditions in which nitro groups are hydrogenated to form amines without producing undesired side reactions. However, traditional processes for hydrogenating nitro groups often yield undesirable side reactions. For instance, nitro compounds have the tendency to form azo compounds during hydrogenation. For example, the hydrogenation of 2-halo-nitrobenzene can result in formation of substantial amounts of
These and other byproducts are often harmful to the environment and are typically expensive to dispose of.
Many important nitro aromatic compounds also include other hydrogenatable groups such as halogens. Dehalogenation has been found to occur when using catalysts made from palladium, platinum, rhodium, nickel and copper chromite. The halogen or other hydrogenatable group is often an important substituent for subsequent reactions or for the final product. Thus, dehalogenation of halonitro aromatic compounds often produces unwanted byproducts. To avoid these undesired byproducts, the nitro group needs to be selectively hydrogenated.
Selective hydrogenation reactions are influenced by many factors. Traditional hydrogenation processes have focused on optimizing conditions by adding catalytic activators, adjusting reaction temperature, adjusting reaction pressure, solvent agitation, and/or modifying other conditions.